The present invention relates to electromechanical filters comprising longitudinally vibrating bars associated with flexurally vibrating couplers. This type of filters is described in U.S. Pat. Nos. 4,163,960 for "Electromechanical filter structure" and 4,197,516 for "Bridged electromechanical filters", both assigned to the same Assignee as the present application, and U.S. patent application Ser. No. 167,702, filed on July 11, 1980 by the Assignee for "Electromechanical filter cells".
The last-mentioned document relates to an elemental cell which has the main advantage of having a band width very near to that of its electrical equivalent, even when the relative band width reaches a value of several tenths.
The present invention has essentially for its object an electromechanical band-pass filter produced by arranging in cascade elemental cells in which the undulation in the transmitted frequency band is reduced with respect to the value of said undulation obtained for filters calculated in accordance with the prior art and based on the theory of electrical filters. It can advantageously be used with filters according to the last-mentioned document and in this case makes it possible to obtain both the desired band width, even a broad band width, and a particularly advantageous transmission characteristic in the band. The realisation of the invention makes it possible to obtain filters, whose residual undulation in a relative band of 20% exceeds by 0.02 dB that of a prototype low frequency filter.
The present invention more particularly relates to the choice of band width of each of the elemental cells constituting the mechanical filter.
For the purposes of the invention, it is assumed that the different stages leading to the obtension of an electromechanical filter are known. Basically, the manufacturer synthesizes an electromechanical filter on the basis of data (central frequency, band width, attenuation out of the band, etc . . . ) defined by a model. The results of the study of the electrical filter are then transported to the mechanical field by applying a system of equivalence between electrical and mechanical magnitudes which are well known in the art. Without going into detail of the theory of filters, it is useful to point out the meaning of certain terms used hereinafter. The theory of filters reduces the study of the desired type filter (e.g. polynominal band-pass) to that of a low-pass filter called a prototype low-pass filter. This prototype filter comprises a cascade of low-pass cells, each being defined by a maximum limiting frequency beyond which transmission is attenuated. The basic electric cells forming the filter having the desired characteristics are obtained from the cells of the prototype filter by a simple mathematical conversion (corresponding to a change of the variable linked with the frequency). By analogy with the cell of the prototype low-pass filter, the band width of a filter cell is defined as the range of frequencies converted from the frequencies limiting the band of the prototype filter cell by the change of variable referred to hereinbefore. In the case of a complete filter obtained by arranging the cells in cascade form, said cells usually being of the same type, the response in the passband is defined by the type of approximation used for the design of the filter (Butterworth, Chebyshev, etc . . . ) on the basis of the attenuation characteristic of the complete filter. The abutment for the attenuation characteristic is the model to be respected. This last condition makes it possible to define, taking account of the selected approximation type, the relationship of the impedances of the branches of the cells of the prototype low-pass filter and, due to the conversion of the variable, that of the cells of the corresponding electrical filter (e.g. band-pass filter). The electromechanical equivalence then makes it possible to obtain the characteristic magnitudes of the equivalent mechanical filter.
As is known, the theory of filters often leads to results which can only be used with great difficulty and it is a conventional practice to make approximations for simplification purposes. As a result of these approximations, real characteristics are obtained which can have an inadmissible deviation compared with the desired characteristic.